Method and apparatus for increasing the sound output of an acoustic transducer

ABSTRACT

Device for increasing the acoustic power radiated in a gas by the solid vibrating surface of a source of sonic or ultrasonic waves, constituted by a flat plate, pierced with openings at regular intervals evenly distributed and arranged in parallel relation disposed at a very short distance from the vibrant surface.

O United States Patent 1111 3,534,1 0

[72] lnventor Sylvlln Jean Janssen [56] References Cited [2'] A I NO $32,3 seim' mm uumzo STATES PATENTS [22] Fiii d v Feb. 17, 1969 2,022,060 11/1935 Swickard 181/32 [45] Patented June 8, 197] 2,567,407 9/1951 Slaymaker 179/110 [73] Assignee Compagnle Des Compteurs FOREIGN PATENTS g gzg 1,175,277 8/1964 Germany 179/180 Pnomy Ftaiiee Primary ExaminerKathleen l-l. Clafi'y [3| 1 Pv 141336 Assistant ExaminerThomas L. Kundert Attorney-Pierce, Scheffler and Parker [54] METHOD AND APPARATUS FOR INCREASING THE SOUND OUTPUT OF AN ACOUSTIC TRANSDUCER 5 chums 4 Drawing Flgs ABSTRACTi Device for increasing the acoustic power [52] U.S. Cl 179/110C, radiated in a as b the solid vibratin surface of a source of 8 Y 8 181/31A sonic or ultrasonic waves, constituted by a flat plate, pierced [51] Int. Cl H04r 15/00 with openings at regular intervals evenl distributed and at- Y [50] Field of Search 179/108, ranged in parallel relation disposed at a very short distance 110.1, 110.3, 110.6, 115, 115.5 ES; 181/31 A,32

from the vibrant surface.

10 0 0 6| a a I z a 0 a1 0 0 a a Q a G a a 0 9 6 a a 44 PATENTED JUN 8 |97| Fi e5.

METHOD AND APPARATUS FOR INCREASING THE SOUND OUTPUT OF AN ACOUSTIC TRANSDUCER The present invention relates to a device for increasing the acoustic power radiated in a gas by the solid vibrating surface of a source of sonic and ultrasonic waves, such as an electrostrictive or magnetostrictive acoustic transducer as well as a process for amplifying such acoustic power.

lt is known that acoustic transducers of this type radiate in gases with, generally, a very poor output. Actually, the amplitude at the solid-gas interface is retained at the level of the vibrating surface of the transducer, so that slight vibratory amplitudes occur during working without destroying the transducer are then found in the gas. The acoustic power, proportional to the acoustic impedance of the medium, that is to say, to the product of the specific mass times the speed of sound in said medium, varies at the solid-gas interface in a ratio of about 1 to l and, consequently, there is only a very slight fraction of power available that is actually utilizable in the gas.

The invention has for an object a device appreciably increasing the sound power radiated in a gas so to obtain an increased vibratory amplitude which, for instance, in the air, can easily reach 5, so that the power radiated, proportional to the square of the vibratory amplitude, is thus multiplied by 25.

The device, according to the invention, is characterized in that it consists of a flat plate, pierced with openings that are at regular distances from each other, and arranged at a very slight distance from the vibrating surface of the acoustic transducer.

This device, for operating correctly, should be kept at about 0.1 mm. from the vibrating surface with an accuracy of about a one-hundredth of 21 mm. The invention provides a process for its operation so as to keep this positioning, in spite of the vibrations and eventual relative displacements of the transducer which prevents the utilizing of a rigid link between this element and the perforate plate.

The invention will be better understood by referring to the description which follows together with the accompanying drawing, given by way of nonrestrictive example, in which:

FIGS. 1 and 2 respectively show a perspective view and a sectional view of a device according to the invention.

FIG. 3 shows the device mounted in a support-frame.

FIG. 4 shows the complete device in its working position.

FIGS. 1 and 2 designate at reference numeral an acoustic transducer of the type described in French Pat. No. 1,429,386 filed Jan. 30, 1965, consisting of a laminated structure of magnetostrictive material and glass. The transducer 10 is provided with an energization winding 11 and supplies, by its vibrating transmitting surface 12, acoustic waves at an ultrasonic frequency of some 10's of Kc./s.

Reference numeral 13 designates a flat plate, made of any kind of material, metallic or otherwise, pierced( with openings 14, of circular shape, and evenly distributed. The plate 13 is held at a very slight distance D from the transmitting face 12, and parallel to said face, by means and according to a process which is explained in that which follows. The distance D is, in general, in the region of 0.1 to 0.2 mm.

During the vibratory movement of the surface 12 of the transducer 10, the gas, situated between this surface and the regions of the plate 13 distant from one of the openings 14, is alternately compressed and relaxed, the viscosity of the gas thus helping to form a series of small compression chambers 15, shown in outline in FIG. 2. Part of the gas can, however, flow through the openings 14 and there is set up in said openings 14, an alternating movement of the gas at the transducer frequency. Thanks to this alternating flow, all the openings 14 radiate in phase towards the exterior, as shown by the arrows, with a maximum acoustic power.

For obtaining a maximum effect, it is advisable to find a compromise between the transmittive surface thus formed by the openings 14 and the leakage of gas through said openings which contributes towards diminishing the compression in the fictitious chambers 15. Also, it is necessary to choose the mean but fixed distance D between the transmitting surface 12 and plate 13.

Experience has shown that for frequencies situated between 18 Kc./s. and 25 Kc./s. and with a magnetostrictive transducer having a transmittive surface of(60 50) mmf radiating in the ambient air, we obtain the best result with a plate having 4 mm. diameter holes, evenly spaced out by 8 mm., the plate being 1.5 mm. thick. The distance D must then be 0.1 mm. With this arrangement, we notice that the acoustic power radiated by the transducer-plate assembly was 20 times greater than that radiated by the face of a transducer alone in the same conditions of electric supply to the transducer.

It is indispensable to ensure a parallelism that is absolutely strict between the transmitting face of the transducer and the flat plate. Moreover, the distance D must be fixed at some hundredths of a mm., for otherwise we shall see a rapid lowering of the vibratory amplitude of the beam radiated by the assembly of openings.

Nevertheless, these openings need not necessarily be of circular shape, and may be replaced by parallel slots, similar to those of a network, provided that a satisfactory ratio R of the remaining area of the plate in relation to the area of the openings alone, be preserved. It has been seen that this ratio R must be preferably comprised between 3 to l and 5 to l to obtain the maximum amplification effect.

We shall now describe the means and method that will enable the rigorous steady maintaining of the distance D between plate and transducer in spite of its vibrations. As a matter of fact, two reasons oppose the simple solution of a rigid mounting of the plate in relation to the transducer. 0n the one hand, when we use a magnetostrictive transducer of high electromechanical output such as that described in French Pat. No. 1,429,386, it is not possible to rigidly fix any kind of solid body by rigid fastenings to the transducer itself, without diminishing the output. In fact, different kinds of parasitic vibrations are set up accompanied by frictions (internal or solid) and the overvoltage coefficient of the transducer resonance appreciably diminishes. Furthermore, the fixing points of the fastenings may most decidedly heat up.

On the other hand, intense vibrations transmitted by solids speedily loosen elements (bolts and nuts or weldings), for fixing and positioning, so that it is practically impossible to speedily position, within about some hundredths of a mm., as wished, the plate in relation to a transducer. One may, at the most, hold and slightly tighten the magnetostrictor by the wire of its energization winding without interfering with its working, but this fixing is far from being rigid and decidedly insufficient for ensuring the strict parallelism of the elements required.

According to another characteristic of the invention, we use, for effecting this positioning, the process employing radiation pressure phenomena that is described in French Pat. No. 1,388,739, filed on Nov. 4, 1963. We know that by accumulating ultrasonic energy between a transmitting surface and a reflector at a short distance (about 0.1 mm. and much shorter than the wavelength of ultrasonic perturbations), considerable repulsion forces are set up between said surface and reflector capable specifically of ensuring the lift of body with a certain surface situated opposite to that of the transmitter. This repulsion force rapidly decreases as a function of the distance D according to the law at 1/D For putting the device of the invention into operation, as shown in FIG. 3, the plate 13 required to be positioned parallel to the transmitting surface 12 of the magnetostrictor 10 is suspended in its plane, OYZ vertical for instance, to a vertical frame 16, by means of spiral springs. The springs 17 only have an equilibrium position around the plate 13 of a slight stiffness in the OX horizontal direction in relation to the stiffness supplied by the repulsion force at l /D.

To position the plate 13 opposite the transmitting surface 12, observing both a rigorous parallelism and a uniform D distance, it is only necessary to approach the magnetostrictor 10 in its working condition, to the plate 13 maintained in the fixed frame 16 and to move the assembly towards the OX direction. in FIG. 4, we see that the plate is thus strictly held parallel to the face 12 of the magnetostrictor, the repulsion forces due to the acoustic radiation pressure producing a resultant force F, opposed to the return force F produced by the springs 17.

The distance D between the plate 13 and the plate 12 varies very slightly with the abscissa A of the plate in relation to the plane of the frame 16 which is kept fixed, and we easily obtain, by acting on the springs 17, the optimal position of the plate with regard to the face 12 which then produces the maximal acoustic energy radiation through the openings 14.

The following computation shows, and this is confirmed by experience, that the influence of the variations of the distance A on the variations of the distance D is very slight: k and K being constant coefficients, the first depending on the springs 17 and able to be slight, the second relating to the radiation pressure and being high, we obtain for the forces F and F the following relations:

F=kA F,=KID And at the equilibrium: kA=I(/D with k/K low.

For each ideal position sought, ensuring a maximum acoustic radiation through the openings in the plate, there corresponds a distance A =K/kD, in that A is much greater than D. Hence, by differentiating: (dD=D/2A) dA with A D, we see that an improper variation 1114, around the ideal position, only involves a slight variation d of D, and the acoustic intensity radiated by transducer-plate assembly hardly varies at all.

The force F being moreover, great with regard to the forces of gravity exerted on the plate, the assembly can be placed in any position, vertical or otherwise, without the relative geometrical arrangement of the two elements being itself altered.

lclaim:

1. A method for increasing the acoustic power radiated in a gas by a solid vibrating surface of a source of sonic and ultrasonic waves by means of a plate provided with a plurality of openings therethrough and resiliently and elastically suspended in a frame surrounding the periphery of said plate comprising moving said frame toward the said vibrating surface of said source so that said plate is parallel to said vibrating surface and continuing such movement until the force exerted by the pressure of the sonic and ultrasonic waves against the plate are balanced by the force exerted by the elastic suspension of the plate in said frame.

2. A device for increasing the acoustic power radiated in a gas by a solid vibrating surface of a source of sonic and ultrasonic waves, comprising a plate having parallel sides and provided with a plurality of free openings therethrough, said plurality of openings being regularly spaced and evenly distributed over the entire surface of the plate, and means for resiliently supporting said plate parallel to the vibrating surface of the source of waves, closely spaced therefrom, and at a substantially constant interval therebetween during use.

3. A device as claimed in claim 2 wherein the plate and the vibrating surface is spaced about 0.1 mm.

4. A device as claimed in claim 2 wherein the means for supporting the plate comprises a frame and resilient elastic means connecting opposed sides of said plate to said frame to exert a force in a direction generally perpendicular to the said vibrating surface whereby the force exerted by the pressure of the waves produced by the said vibrating surface of the source against said plate is balanced by said resilient elastic means.

5. A device as claimed in claim 2 wherein the openings are so provided in the plate that the ratio of the area of said plate less the area of the openings to the area of the openings is within the range from about 3:1 to about 5:1. 

1. A method for increasing the acoustic power radiated in a gas by a solid vibrating surface of a source of sonic and ultrasonic waves by means of a plate provided with a plurality of openings therethrough and resiliently and elastically suspended in a frame surrounding the periphery of said plate comprising moving said frame toward the said vibrating surface of said source so that said plate is parallel to said vibrating surface and continuing such movement until the force exerted by the pressure of the sonic and ultrasonic waves against the plate are balanced by the force exerted by the elastic suspension of the plate in said frame.
 2. A device for increasing the acoustic power radiated in a gas by a solid vibrating surface of a source of sonic and ultrasonic waves, comprising a plate having parallel sides and provided with a plurality of free openings therethrough, said plurality of openings being regularly spaced and evenly distributed over the entire surface of the plate, and means for resiliently supporting said plate parallel to the vibrating surface of the source of waves, closely spaced therefrom, and at a substantially constant interval therebetween during use.
 3. A device as claimed in claim 2 wherein the plate and the vibrating surface is spaced about 0.1 mm.
 4. A device as claimed in claim 2 wherein the means for supporting the plate comprises a frame and resilient elastic means connecting opposed sides of said plate to said frame to exert a force in a direction generally perpendicular to the said vibrating surface whereby the force exerted by the pressure of the waves produced by the said vibrating surface of the source against said plate is balanced by said resilient elastic means.
 5. A device as claimed in claim 2 wherein the openings are so provided in the plate that the ratio of the area of said plate less the area of the openings to the area of the openings is within the range from about 3:1 to about 5:1. 